On the physical origin of OVI absorption-line systems

We present a unified analysis of the O VI absorption lines seen in the disk and halo of the Milky Way, high-velocity clouds, the Magellanic Clouds, starburst galaxies, and the intergalactic medium. We show that these disparate systems de ne a simple relationship between the O VI column density and absorption-line width that is independent of the oxygen abundance over the range O/H similar to 10% to twice solar. We show that this relation is exactly that predicted theoretically as a radiatively cooling flow of hot gas passes through the coronal temperature regime-independent of its density or metallicity (for O/Hgreater than or similar to0.1 solar). Since most of the intergalactic O VI clouds obey this relation, we infer that they cannot have metallicities less than a few percent solar. In order to be able to cool radiatively in less than a Hubble time, the intergalactic clouds must be smaller than similar to1 Mpc in size. The implied global heating rate of the warm/hot intergalactic medium is consistent with available sources. We show that the cooling column densities for the O IV O V Ne V and Ne VI ions are comparable to those seen in O VI This is also true for the Li-like ions Ne VIII Mg x and Si XII (if the gas is cooling from Tgreater than or similar to10(6) K). All these ions have strong resonance lines in the extreme-ultraviolet spectral range and would be accessible to Far Ultraviolet Spectroscopic Explorer at zgreater than or similar to0.2-0.8. We also show that the Li-like ions can be used to probe radiatively cooling gas at temperatures 1 order of magnitude higher than where their ionic fraction peaks. We calculate that the H-like ( He-like) O, Ne, Mg, Si, and S ions have cooling columns of similar to10(17) (a few times 10(16)) cm(-2). The properties of the O VII O VIII and Ne IX X-ray absorption lines toward PKS 2155-304 may be consistent with a scenario of radiatively cooling gas in the Galactic disk or halo.